1. Field of the Invention
The present invention relates to a ceramic composition and a multilayer ceramic capacitor made therefrom for electronic machines and equipment, and more particularly to a multilayer ceramic capacitor having internal electrodes of nickel or nickel alloy.
2. Description of the Prior Art
Multilayer ceramic capacitors are produced conventionally by a process which consists of the following steps. First, a dielectric ceramic material is prepared which is in the form of sheet with a surface coating material to be converted later into an internal electrode. This dielectric ceramic material is composed mainly of BaTiO.sub.3. Second, several pieces of coated sheets are stacked and bonded together by heating under pressure. The resulting monolithic assembly is fired at 1250.degree.-1350.degree. C., thereby giving a consolidated stack of dielectric ceramic layers having internal electrodes. Finally, the end faces of the stack are provided with external electrodes which communicate with the internal electrodes.
The above-mentioned process requires that the internal electrodes be made of a material which satisfies the following two conditions.
(a) It should have a melting point higher than the firing temperature of the dielectric ceramic material because the internal electrodes are formed simultaneously with the firing of the dielectric ceramic material.
(b) It should not oxidize even in an oxidizing atmosphere at high temperature, nor should it react with the dielectric ceramic material.
This requirement is met conventionally by making electrodes from noble metal such as platinum, gold, palladium and silver-palladium alloy. Unfortunately, they are expensive, albeit superior in characteristic properties. This causes the material for electrodes to account for a large portion in the cost of multilayer ceramic capacitor and hence is the prime factor in increasing the production cost.
Besides noble metals, there are high-melting base metals such as Ni, Co, W, and Mo. Unfortunately, they easily oxidize in an oxidizing atmosphere at high temperature, and fail to function as an electrode. For these base metals to be used as internal electrodes of a multilayer ceramic capacitor, it is necessary that they be fired, together with dielectric ceramics, in a neutral or reducing atmosphere. Unfortunately, conventional ceramic materials undergo substantial reduction to become a semiconductor upon firing in such an atmosphere.
In order to eliminate this disadvantage, there have been proposed new ceramic materials as disclosed in Japanese Patent Publication No. 42588/1982 and Japanese Patent Laid-open No. 101359/1986. The first is characterized by the solid solution of barium titanate containing barium and titanium in a ratio greater than the stoichiometric one. The second is characterized by the solid solution of barium titanate with rare earth oxides such as La.sub.2 O.sub.3, Nd.sub.2 O.sub.3, Sm.sub.2 O.sub.3, Dy.sub.2 O.sub.3 and Y.sub.2 O.sub.3.
There have also been proposed new dielectric ceramic materials less liable to change in permittivity with temperature, as disclosed in Japanese Patent Laid-open No. 256422/1987 and Japanese Patent Publication No. 14611/1986. The former has a composition of BaTiO.sub.3 -CaZrO.sub.3 --MnO--MgO, and the latter has a composition of BaTiO.sub.3 --(Mg, Zn, Sr, Ca)O--B.sub.2 O.sub.3 --SiO.sub.2. These dielectric ceramic materials do not become a semiconductor even on firing in a reducing atmosphere. Thus they permit the production of multilayer ceramic capacitors having internal electrodes of basic metal such as nickel.
Recent developments in electronics has promoted the size reduction of electronic parts, and there is a marked trend for multilayer ceramic capacitors to become smaller in size and larger in capacity, with the dielectric ceramic material increasing in permittivity and the dielectric ceramic layers decreasing in thickness. This has aroused a growing demand for reliable dielectric ceramic materials which have a high permittivity and a small change with temperature in permittivity.
The requirement for high permittivity is met by the above-mentioned dielectric ceramic materials (disclosed in Japanese Patent Publication No. 42588/1982 and Japanese Patent Laid-open No. 101359/1986). However, they suffer the disadvantage of giving rise to a dielectric ceramic having such large crystal grains that the number of crystal grains present in one layer is so small as to impair reliability in the case where the dielectric ceramic layer is thinner than 10 .mu.m. Moreover, they greatly change in permittivity with temperature. Thus they have not yet gained market acceptance.
These disadvantages are overcome by the above-mentioned dielectric ceramic material (disclosed in Japanese Patent Laid-open No. 256422/1987) which has a comparatively high permittivity, gives rise to a dielectric ceramic with small crystal grains, and is less liable to change in permittivity with temperature. However, it has a problem with reliability at high temperatures because CaZrO.sub.3 and CaTiO.sub.3 (which appear during firing) easily form the secondary phase together with Mn.
The above-mentioned dielectric ceramic material (disclosed in Japanese Patent Publication No. 14611/1986) gives rise to a dielectric ceramic which has a permittivity of 2000-2800 and hence it is unfavorable for producing multilayer ceramic capacitors with a reduced size and an increased capacity. In addition, it does not meet the EIA standards (X7R) which provides that the change in electrostatic capacity at temperatures ranging from -55.degree. C. to +125.degree. C. should be within .+-.15%.
Moreover, there is disclosed a non-reducing dielectric ceramic in Japanese Patent Laid-open No. 103861/1988. It suffers the disadvantage that it greatly changes in insulation resistance and capacity with temperature depending on the crystal grain size of BaTiO.sub.3, its principal component. This poses difficulties in control for stable characteristic properties. In addition, it is not of practical use because it has an insulation resistance and an electrostatic capacity such that their product (CR) is 1000-2000 M.OMEGA..multidot..mu.F.
In order to address the above-mentioned problems, there have been proposed a variety of compositions in Japanese Patent Laid-open Nos. 9066/1993, 9067/1993, and 9068/1993. However, even they do not meet the recent stringent requirements for small size, large capacity and thin layer, as well as reliability.
Reliability greatly decreases if the thickness of the dielectric ceramic layer is simply reduced, with the rated voltage kept constant, because this increases the electric field strength per layer and lowers the insulation resistance at room temperature and high temperatures. Therefore, it is necessary in the case of conventional dielectric ceramic materials, to lower the rated voltage if the thickness of the dielectric ceramic layer is to be reduced.
The foregoing has caused a demand for a highly reliable multilayer ceramic capacitor which maintains the rated voltage despite the dielectric ceramic layers with a reduced thickness and has a high insulation resistance in a strong electric field.
In the meantime, it is common practice in the case of small-size, large-capacity multilayer ceramic capacitors, to coat their external electrodes (which can be formed by sintering from electrically conductive metal powder) with a plated film so as to facilitate their surface mounting. The plated film is formed usually by electrolytic plating. However, this practice impairs reliability because when multilayer ceramic capacitors are immersed in a plating solution to form a plated film, the plating solution infiltrates into minute voids in the electrodes (which occur when electrically conductive metal powder is sintered), reaching the interface between the internal electrode and the dielectric ceramic layer.